This invention relates to a signal processing system and more specifically to an amplitude range matching system.
An amplitude range matching system, which can be implemented in analog or digital form, processes the amplitude range of an information signal which contains amplitude information within an amplitude range and frequency information within a frequency range. The lowest possible information signal amplitude may be defined as the lower limit of the information signal amplitude range, which the highest possible information signal amplitude may be defined as the upper limit of the information signal amplitude range. The information signal amplitude range is, therefore, the range over which the information signal amplitude can vary between its lower and upper limits, and is typically measured in Volts. Similarly the lowest possible information signal frequency may be defined as the lower limit of the information signal frequency range and the highest possible information signal frequency being defined as the upper limit of the information signal frequency range. The information signal frequency range is, therefore, the range over which the Information Signal frequency can vary between its lower and upper limits, and is typically measured in Hertz. Amplitude range matching systems are employed to improve the clarity of reception of the information signal by matching the processed information signal amplitude range to the optimum amplitude range of the information signal receptor, which may be human or electronic. Over this optimum amplitude range reception sensitivity is at a maximum and information signal amplitudes may be clearly received without causing distortion or damage to the receptor. This process does not affect the frequency content of the information signal within the information signal frequency range so that the processed information signal contains the same frequency content as the information signal.
A receptor which has a limited optimum amplitude range cannot receive all of the information contained within an information signal which has a larger amplitude range. Similarly, if a receptor has an optimum amplitude range which is larger than the information signal amplitude range then lack of reception sensitivity could result in the loss of the detailed information contained within the information signal during reception. Both these situations require a system which can match the information signal amplitude range to the optimum amplitude range of the receptor.
Many techniques have been developed to receive information within a defined frequency range. There are at least three main problems associated with receiving the information held within a defined amplitude range: (1) lack of reception clarity due to a poor signal to noise ratio; (2) loss of information; and (3) potential damage to the receptor. These problems are caused when the amplitude range of the information signal does not match with the optimum amplitude range of the receptor. For example, if a receptor has a fixed amplitude range it will not be able to receive all the information within a signal which has a larger amplitude range and may be damaged by the information signal. If a receptor has a fixed amplitude range which is much larger than the amplitude range of the information signal then detailed information may be lost due to the lack of sensitivity of the receptor as encountered in the United Kingdom Patent No. GB 2,224,413A. Human, electronic, electrical and mechanical receptors generally have a fixed optimum amplitude range over which they operate comfortably and at maximum sensitivity.
Prior art techniques employ linear gain variation arrangements to increase the low amplitude portion of an information signal. In such arrangements, however, the high amplitude portion of the information signal is amplified outside the upper limit of the optimum amplitude range of the receptor causing loss of information and possibly causing damage to the receptor. If the same linear gain variation is used to decrease the high amplitude portion of the information signal then the low amplitude portion of the information signal will be attenuated outside the lower limit of the optimum amplitude range of the receptor causing loss of information. Linear gain variation has the same effect on both the signal and the noise and cannot be used, therefore, to improve the signal to noise ratio in the processed information signal which results in a reduction in the reception clarity.
Automatic gain control and fast fourier analysis normalization techniques can be used for amplitude range matching but often introduce switching noise into the processed information signal, where switching noise, in the present context, is a sudden large change in the amplitude of the processed information signal due to a gain change which is not sufficiently controlled.
Averaging techniques which set the gain applied to the information signal based on the average of the information signal amplitude over a certain time period introduce a delay into the application of gain changes on the information signal. This may result in low amplitude signals being lost due to insufficient gain, high amplitude signals causing distortion or damage to the receptor due to excessive gain and the introduction of switching noise into the processed information signal due to a gain change being carried out too slowly.
Some prior art systems have employed compression techniques. However, such compression techniques can introduce distortion into the processed information signal due to excessive gain on low level information signals and excessive attenuation of high level information signals so that the detail in the processed information signal may be lost.
Thus there is a need for a system that matches the information signal amplitude range to the optimum amplitude range of the receptor, without introducing switching noise into the processed information signal, reducing reception clarity, causing loss of information or potential damage to the receptor. There is also a need for an amplitude range matching system that can be used to process information signals of any frequency range.
The present invention, according to one embodiment, relates to a method for matching the dynamic range of an information signal to the dynamic range of a receptor. The method comprises the steps of: monitoring the information signal; varying the gain of the information signal in response to the monitoring step so as to provide a gain adjusted information signal; generating a variable control signal having a frequency outside the frequency range of the information signal, in response to the monitoring step; combining the control signal with the gain adjusted information signal so as to provide a combined information signal; and providing a variable amplitude dependent gain to the combined information signal so as to provide an output signal that substantially matches the dynamic range of the receptor.
According to one embodiment, wherein prior to the step of generating a variable control signal, the method further comprises a first filtering step for filtering out frequencies outside the frequency range of the information signal. The method may also comprise the step of providing a linear amplification to the information signal.
According to still another embodiment of the invention, the combining step of the method further comprises the step of varying the ratio of the control signal and the gain adjusted information signal. This ratio may be varied so as to equal a ratio of 1:1. Furthermore, the step of providing a variable amplitude dependent gain may further comprise the step of subjecting the combined signal to a power of 1/n, wherein n is a positive number including a fraction, so as to provide a combination of variable compression and expansion factors. The method may also comprise a second filtering step for filtering out all frequencies outside the frequency range of said information signal, after which may be performed the step of providing a linear amplification.